Quantification of sequestered fossil-derived CO₂ in cementitious materials and its atmospheric contamination using carbon isotope measurements

Carbonation of cementitious materials has attracted increasing attention as a potential approach for CO₂ fixation. However, quantitative evaluation of the amount of CO₂ fixed from a designated source under accelerated carbonation conditions remains challenging, because atmospheric CO₂ mixing may occur. In this study, carbonation experiments were conducted on well-hydrated cement paste powders under controlled relative humidity conditions using fossil-derived CO₂, and carbon uptake and isotopic compositions were investigated using combined 13 C and 14 C analyses. The results show that atmospheric CO₂ can contribute significantly to the isotopic composition of the fixed carbon in the samples. Two approaches were proposed to quantify this atmospheric contribution, which yield contamination estimates of comparable magnitude. Based on the estimated atmospheric mixing, a practical framework is established to quantify the amount of CO₂ fixed from the 14 C-free fossil-derived source gas. The proposed approach enables source-specific evaluation of CO₂ fixation in cementitious materials and supports quantitative assessment and labeling of carbon-captured materials.